libvhost-user: Fix VHOST_USER_GET_MAX_MEM_SLOTS reply

libvhost-user: Fix VHOST_USER_GET_MAX_MEM_SLOTS reply

With REPLY_NEEDED, libvhost-user sends both the acutal result and an
additional ACK reply for VHOST_USER_GET_MAX_MEM_SLOTS. This is
incorrect, the spec mandates that it behave the same with and without
REPLY_NEEDED because it always sends a reply.

Fixes: 6fb2e173d20c9bbb5466183d33a3ad7dcd0375fa
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20220627134500.94842-3-kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>